1. Field of the Invention
This invention relates to the deposition of a metal as a patterned interlayer in a matrix.
2. Background
There exist in the art many different chemical or physical methods by which a zero-valent metal can be introduced into or placed on a polymeric film. Among these, some are capable of producing a thin continuous coating at a surface of the polymer, for example, metal vapor deposition or electroless plating. Other processes, such as silver halide photography, discussed further below, and certain forms of chemical deposition, produce metal particles embedded within the polymeric film. These particles are generally dispersed to such a degree that they lack the characteristic electrical or optical properties of a continuous metal layer.
Existing art includes two ways in which metals can be introduced into preexisting solid polymeric films. The most common example involves photographic technology (see, for example, A. Roff and E. Weyde, "Photographic Silver Halide Diffusion Process", The Focal Press, London and New York, 1972, pp 13-31). Conventional photographic methods produce metal particles dispersed within a polymeric film, wherein their primary purpose is to absorb light. These particles are generally dispersed to such a degree that they do not constitute a continuous phase and, therefore, lack the macroscopic properties of electrical conductivity and optical reflectivity commonly associated with surface-metallized structures.
The second method of introducing a metal into a preexisting solid polymeric film is that of U.S. Pat. No. 4,512,855 which discloses a metal interlayer deposition (MID) process. This process involves reduction of metal ions, diffusing from one surface of a polymeric film, by electrons supplied via redox reactions of the polymer, the electrons being supplied from the opposite surface of the film. Among the unique and desirable features of the MID process is the fact that it is capable of producing metal interlayers of sufficient density that they are electrically conductive, and with sufficiently smooth and sharply defined surfaces that they are optically reflective.
Additional techniques for introducing metallic interlayers into preexisting films are disclosed in U.S. Pat. No. 4,657,833 and in commonly assigned pending U.S. patent applications Ser. Nos. 820,234 filed Jan. 21, 1986 and allowed Apr. 23, 1987; and 901,770 filed Aug. 29, 1986 and allowed Dec. 18, 1986. The disclosures of U.S. Pat. Nos. 4,512,855 and 4,657,833 and the aforesaid patent applications are hereby incorporated by reference. The techniques disclosed therein are of use in carrying out the invention which will be described in detail hereinafter.
The techniques of the previous two paragraphs are dependent on the diffusion of metal ions (M.sup.n+) through the film from one surface thereof. Some of the techniques are dependent on the diffusion of a solution of a reducing agent through the film from an opposite surface of the film. The techniques of U.S. Pat. Nos. 4,512,855 and 4,657,833 are notably different from the other techniques in that, although metal ions (M.sup.n+) diffuse from one surface of the film, the reduction of the metal ions to zero-valent metal is effected by means of electrons which do not diffuse in solution through the film, but rather are transported through the film itself. The films of these patents comprise a reversibly redox active polymer, that is, a polymer which is capable of accepting and donating electrons rapidly and without competing, irreversible chemical changes. Although diffusion of a solution of a reducing agent through a polymer is different mechanistically from the transporting of electrons through a reversibly redox active polymer, and although reduction of metal ions by a reducing agent in solution and reduction of metal ions by electrons in a polymeric matrix are different in kind, the invention which is described in detail hereinafter is applicable to both systems, that is, wherein metal ions are reduced either by a reducing agent in solution or by electrons in a polymeric matrix. For this reason, the term "counter-current diffusion" is used herein in the generic sense and includes both electro-chemical metal interlayer and chemical metal interlayer formation techniques. Moreover, the term "reductant" (R), as used herein, embraces both electrons which are transported in a reversibly redox active polymer, and chemical reducing agents which are transported in a solution. The metallic interlayers formed by the above techniques can be conductive, thereby being useful in the formation of conductive sheets. Furthermore, patterned metallic interlayers can be useful as flexible circuits and in other circuitry applications, including partially transparent electronic shielding. These patterned metallic interlayers can be formed via the techniques cited in the previous paragraph by controlling the access of either M.sup.n+ or R to certain regions along the surface of the film. There are three general methods of controlling the access of M.sup.n+ or R at the film surface;
1. A reactant barrier mask placed at the surface of the film limits the areas where reactants can diffuse into the film. PA0 2. A patterned cathode defines the area where electrons are available at the surface of the film. PA0 3. The electrons from a cathode are allowed access to the film in a two-dimensionally controlled fashion by a photosensitive layer. PA0 (a) a process by means of which zero-valent metal can be deposited within a matrix; PA0 (b) a process by means of which, if desired, the metal can be deposited as an interlayer within the boundaries of the matrix surfaces; PA0 (c) a process by means of which the position of the interlayer in the matrix can be systematically controlled so as to be any finite distance below the surface of the matrix; PA0 (d) a process by which multiple interlayers can be deposited within a single matrix in a systematically controlled manner; PA0 (e) control over the width of the resultant interlayer by changing the chemical reaction rate between the metal ion and the reductant (R); PA0 (f) a process which, if desired, will yield a metal interlayer having sufficient density, continuity and surface regularity to exhibit electrical conductivity and optical reflectivity which are characteristic of bulk metallic phases or surface coatings; and PA0 (g) a process that does not require an electroactive polymeric matrix, that can be carried out in a single step on a free standing matrix of variable geometry, and that can be used to produce interlayers having sharp boundaries so as to make the matrix more reflective on both sides. These and other objects will become apparent hereinafter. PA0 (1) selective thermal treatment of the matrix, and PA0 (2) photochemically induced formation of an interpenetrating polymer network. The polymerizable additive can be any that substantially reduces matrix swelling. Polyfunctional monomers are a preferred and exemplified subset.
The first method can be used with all of the counter-current diffusion methods described above. The reactant barrier mask can be any material which is impermeable to one of the reactants. The second method is only applicable to counter-current diffusion methods wherein at least one of the reactants is supplied by an electrode at the surface of the film. All of the patterning methods which limit reactant access at the film surface suffer inherent resolution losses due to lateral diffusion of the reactants in the matrix after passing the surface barrier.
Many methods have been developed to create conductive images on a variety of substrates. Most of the imaging methods rely on the application of a mask to the substrate, which mask physically protects the desired areas during the metallization process. For low resolution applications the mask can be pressed against the substrate during metallization, as is done in screen process printing. For higher resolution applications a resist material which is sensitive to electromagnetic radiation is applied to the substrate, and the desired pattern is imaged through an appropriate mask or written with a collimated source of electromagnetic radiation (R. C. Daly and J. L. R. Williams, Polymer News, 11, 164 (1986)). Recently, an electroplating technique has been developed for generating three dimensional metallic surface features by spatially modulating the current density through a variable thickness polymeric film (J. C. Angus, U. Landau, S. H. Liao and M. C. Yang, Journal of the Electrochemical Society, 133, 1152 (1986); Koontz et al., U.S. Pat. No. 4,001,093; and Angus et al., U.S. Pat. No. 4,361,641). In this process the thickness of a gel film applied to the desired substrate is controlled by photochemical crosslinking of the gel film through a mask.
The formation of a photoimaged interpenetrating polymer network (IPN) in polyamic acid has been previously used to control polyamic acid solubility, and thereby create polyimide photoresist patterns (L. Minnema and J. M. van der Zande, Abstracts 30th IUPAC International Symposium on Macromolecules, 224 (1985)). This art does not disclose or suggest the use of a photoimaged polymer network to control permeability.
U.S. Pat. Nos. 3,718,473 and 3,879,204 disclose processes for making gravure printing plate resists, in which processes a photopolymerizable layer is rendered resistant to diffusion of an aqueous etchant when exposed to light. The exposed layer is then placed on a metal sheet which is exposed to an etching solution which produces a continuous tone gravure which is suitable for printing. Unlike the invention which will be described hereinbelow, the systems of the patents involve a two-phase dispersion of monomer, and the surface resist which is produced is not itself a final product, but rather is used to produce a final product, namely, a print. Moreover, there is no disclosure or suggestion in the patents that the extent of swelling, which affects the degree of permeability, can be controlled.
Many factors can affect the diffusive transport of reactants in a swollen matrix, including the size of the reactants and the degree of swelling of the matrix. Many theoretical analyses of these factors have been developed, for example, N. J. Peppas and H. J. Moynihan, Journal of Applied Polymer Science, 30, 2589 (1985). The important pertinent observation is that the permeability of reactants decreases as the degree of matrix swelling is reduced.
It is an object of this invention to provide a process by which zero-valent metal can be deposited and patterned in a sharp two-dimensional fashion within a matrix, for example, a film, such as a polymeric film, for example, an organic polymeric film, by use of diffusion controlled metal deposition processes. Further objects include and provide: